1. Field of the Invention
This invention relates to pulse radar apparatus and to receivers therefor and is concerned more particularly with radar apparatus for the detection of surface targets, for example, marine radar apparatus.
2. Prior Art
It is known to use pulse length discrimination as an anti-clutter and anti-interference circuit in receivers of pulse radar equipment. Pulse length discriminators for example are described in:
Nathanson, F. E. Radar Design Principles -- Signal Processing and the Environment, McGraw-Hill, 1969, pages 112-119;
Lawson, J. L. and G. E. Uhlenbech, Threshold Signals -- MIT Radiation Laboratory Series No. 24. McGraw-Hill 1950; and
Hansen, V. G. Studies of Logarithmic Radar Receivers Using Pulse-length Discrimination. IEEE Trans. Vol. AES-1, No. 3. December 1965.
Such pulse length discriminators are analogue devices and, because they are amplitude conscious, to use such devices with any degree of success in practice, some form of limiting of the IF or video amplitude is necessary. It is therefore an established technique to use low gain IF amplifiers with as good as dynamic range as possible followed by logarithmic video amplifiers or, better still, to use one of the forms of logarithmic IF amplifiers followed by a video high pass filter, that is to say, a fast time constant circuit. This combination, in a microwave radar receiver, of a mixer/local oscillator followed by a preamplifier in a narrow band IF filter and then a wide band logarthimic IF amplifier strip with built-in video detectors driving a fast time constant circuit gives a good constant false alarm rate (CFAR) characteristic in Rayleigh distributed clutter from sea or rain or from chaff in a hostile environment. Under these conditions, if a fast time constant circuit is replaced by a pulse length discriminator matched to the pulse length, a degree of receiver selectivity takes place, not only in the frequency domain due to the post preamplifier narrow band filter, but also in the rejection of interfering pulses which are either longer or shorter than the transmitter pulse length.
Unfortunately echoes from range-extensive targets are rejected by the pulse length discriminator as these result in the received signals being longer than those transmitted. On a marine radar, the range resolving power of even a modest resolution radar (for example one having a transmitted pulse length of 0.1 microseconds) is of the order of 15 meters. Many ships exceed this length and hence a pulse length discriminator as described above would reject responses from most ships. For this reason, pulse length discriminators have been used mostly in ground-to-air or air-to-air radars which use a longer pulse length than marine radars, typically one microsecond. A one microsecond pulse is equivalent to 150 meters range resolution and hence there is negligible lengthening of received pulses from aircraft targets. However there is a growing need in marine radars to be able to detect small craft, for example when searching for small patrol craft. Pulse length discrimination can help in the detection of small targets but, with marine radars, a further pulse distortion mechanism has to be taken into account. The radar signal reflected directly from a target which is received by a ship-borne radar is contaminated by the indirect signals due to sea reflection. Measurement has shown that, even with a one-way beacon transmitting system, the transmission of a pulse of 0.5 microseconds can be distorted to produce a pulse varying from 0.5 to 0.8 microseconds. There is evidence moreover in some cases of a reduction in duration due to the sea reflected signal so that the original 0.5 microsecond pulse is reduced to considerably less than that time period. For this reason, a pulse length discriminator operating on a sea-borne radar with high discrimination of unwanted pulse lengths would be accompanied by a serious loss of wanted targets. This is due, firstly, to many of the targets of interest having a size exceeding the range resolution of the radar set, particularly bearing in mind that pulse lengths as short as 0.05 microseconds are now commonly in use. Secondly, point targets will return a generally lengthened but possibly shortened pulse due to the above described sea reflection and multipath phenomena. For these reasons, the use of pulse length discrimination for clutter and interference rejection in marine radars has heretofore been strictly limited; it has generally been considered preferable to use other forms of discrimination, for example integration at the pulse recurrence frequency, for clutter and interference rejection.